The worldwide interest in high performance extrudated materials of renewable origin is increasing due to an accelerating demand of environmentally friendly materials for industrial use and also due to the depletion of petroleum resources. Wheat gluten (WG) is one of the candidates contemplated to replace petroleum-based oxygen-barrier polymers for packaging applications. It is a low-cost by-product, from the increasing bio-fuel (ethanol) production, and WG films generally show low oxygen permeability at dry conditions, due to a high content of hydrogen bonds.
The extrudate quality of WG films depends on several parameters that can be controlled to various extents, including applied mechanical energy, shear impact, applied pressure, plasticizer type and content, processing time and temperature. These parameters influence conformational changes, chemical crosslinking and protein aggregation that occur during processing. Sulfhydryl, on the cystein amino acid, is responsible for the creation of disulfide crosslinks during oxidation. An important part of the protein aggregation is a reorganization of the intramolecular disulfide bonds to intermolecular disulfide bonds via thiol-disulfide exchange reactions. The process-induced increase in molar mass, number of chain entanglements and decrease in solubility have been reported in a number of papers.
The processing temperature is one of the main factors that control the protein thiol-disulfide exchange reactions. The upper processing temperature, related to the onset of aggregation, can be increased through limitation of the amount of disulfide-crosslink reactions taking place. It has been shown that it is possible to increase the extrusion temperature using salicylic acid, possibly by reducing and/or delaying disulfide crosslinking through radical scavenging.1 For solution cast systems, pH is also well known to affect film forming properties. As an example, at pH 7.5, which is close to the WG isoelectric point, it is difficult to produce films. Generally, WG films obtained at pH 2-4 and pH 9-13 are relatively homogenous, while at pH 5-6 they are of poor quality and at pH 7-8 they do not form at all.
For optimal strength and barrier properties, and to obtain desired transparent/translucent properties, as the case may be, the film extrudates have to be homogenous, such that protein particles readily fuse together.
Continuous processing of a plant protein, such as wheat gluten, is difficult without the use of solvents, and often yields a grainy-type of extrudate with inferior mechanical and barrier properties. The use of solvents necessitates an energy-consuming drying step, and renders high temperatures in the extruder impossible. Consequently, there exists a need for an improved process for manufacturing extrudates based on plant protein(s).